1. Field of the Invention
This invention is concerned with an improved process for deasphalting asphalt-containing hydrocarbon oils. In particular, it is concerned with solvent-deasphalting of residual oils with a liquid solvent composition that contains at least 10 volume percent of hydrogen sulfide or carbon dioxide and propane.
2. Description of the Prior Art
Atmospheric or vacuum distillation of petroleum oils produces a residual fraction which may represent, depending on the crude, as much as 50 volume percent of the crude charged. With suitable crudes, particularly those designated paraffinic or naphthenic, the residuum can be processed to yield lubricant base stock oils. Often, however, the residuum is not so suited and must be converted by coking or disposed of as low-grade fuel.
In any case, the residuum from atmospheric or vacuum distillation contains the non-volatile asphaltene fraction of the crude as well as almost all of the metal contaminants and a large fraction of the sulfur content of the whole crude. These asphaltic, metal and sulfur constituents are generally regarded as impurities, and in general they detrimentally affect processes that might otherwise be used to convert the residual oil to more desirable products. The asphaltene fraction, for example, which is present in a dispersed state in the crude, tends to flocculate with the deposition of insoluble coke or sludge, especially on heating. Thus, this impurity is undesirable in high grade lubricants and must be removed. The use of an untreated residual oil as chargestock to the catalytic cracking process is detrimental to efficient operation because the nickel and vanadium impurities present in the oil deposit and build up on the cracking catalyst with loss of selectivity and even loss of plant capacity in many cases. Catalytic demetallation processes conducted in the presence of added hydrogen have been proposed for removing metals from residual oils, but these processes in general have minimal effect on the Conradson Carbon value which is usually directly related to asphaltene content. Thus, even with catalytically demetallized residual oil, use of such oil as feedstock to a catalytic cracker to convert it to gasoline results in a high coke load on the regenerator, which in turn limits the throughout capacity of the cracking unit. Thus, it is evidently desirable to remove the asphaltene fraction from residual oils with reduction of the other contaminants if possible, and with production of high grade asphalt by-product.
Solvent-deasphalting of residuum is well known in the art and many solvents and solvent combinations have been suggested for this process. Most commonly, light hydrocarbon solvents containing 3 to 5 carbon atoms in the molecule such as propane, propylene, butene, butane, pentene, pentane and mixtures thereof and used either alone or in admixture with other solvents such as ketones, liquid SO.sub.2, cresol and diethyl carbonate. Typical of prior art deasphalting processes is the process described in U.S. Pat. No. 2,337,448 in which a heavy residuum is deasphalted by contacting it at elevated temperature with a deasphalting solvent such as ethane, ethylene, propane, propylene, butane, butylene, isobutane, and mixtures thereof. The entire contents of the U.S. Pat. No. 2,337,448 are incorporated herein by reference.
The term "deasphalting" is used herein in the conventional sense, i.e. to signify a process wherein a tar fraction containing asphaltenes is removed from an oil. This tar fraction contains the bulk of the metal-bearing porphyrins contained in the crude. Thus, the deasphalting process is also a demetallizing process, and is to be so understood herein.
Propane is the solvent most often used in deasphalting operations. However, propane-deasphalting is somewhat limited in that it will extract only about 40 to 60 percent of a petroleum residuum, and the tar fraction resulting from this process, which amounts to about half of the residuum, is unsuitable for use except as an ingredient in the blending and production of heavy fuel oils. Additional refining treatments must be employed with the tar fraction in order to more effectively separate the asphalt and release additional deasphalted oil from this fraction. Generally, use of the higher molecular weight aliphatic hydrocarbons such as butane and pentane will result in a greater yield of deasphalted oil and produce asphalt with a higher softening point. However, as one uses solvent of increasing molecular weight and/or boiling point, one loses the advantage of the facile stripping under mild conditions that is obtainable with the low-boiling propane.
U.S. Pat. No. 4,054,512 to Dugan et al teaches the use of liquid hydrogen sulfide as the deasphalting solvent. That metals may be selectively removed from petroleum residues by extraction with supercritical gases such as propane and carbon dioxide is disclosed by Solomon, Abstracts, Paper No. 28d, page 38, 70th National Meeting, American Institute of Chemical Engineers, Atlantic City, Aug. 29-Sept. 1, 1971.
It is an object of this invention to provide an improved solvent-deasphalting process for treating asphalt-containing mineral oils. It is a further object of this invention to provide novel, highly selective solvent combinations for recovering deasphalted oil and high quality asphalt from a residual petroleum oil. It is a further object of this invention to demetallize and deasphalt residual oils for use as chargestock to the fluid catalytic cracking process. Other objects will be evident from reading this entire specification including the claims thereof.